Wood is not strictly a single fuel. It is a mixture of solids, liquids and gases. Wood burns in three distinct phases, which may proceed in sequence or all at once. In the first phase, free water is driven out of the wood. The water is boiled out by the heat of the fire, thus a lot of calories are used up, which would otherwise be used as heat for a home. A lot of energy is wasted in phase one.
The heat from phases two and three is what powers phase one. In phase two, the wood begins to break down into charcoal, gas, and voliatile liquids. As much as 50% of the potential heat inside a log is released by the burning of the gas and voliatile liquids. However, most of this potential heat escapes up the flue.
In phase three, combustion of the charcoal takes place. It is the charcoal that provides most of the useable heat from a conventional stove. However, as the wood turns to ashes, the ashes smother the charcoal.
Research has shown that a two inch bed of coals will consume all the oxygen in the air admitted into a stove, under normal draft conditions. In this case, almost all of the carbon is being oxidized fully into carbon dioxide. This is ideal. But if the bed of coals is thicker--say three inches--the ideal of complete combustion is completely lost. Now a very large amount of carbon monoxide is produced, and this half-burned carbon will pass up the flue. Half-burned carbon means less heat produced, less efficiency, and a waste of wood.
Modern wood stoves have been dealing with CO emissions by introducing what they call secondary air intakes. This they claim, introduces extra oxygen so the CO can be converted to CO.sub.2. Also, the secondary air allows the other volatiles to ignite and burn, thus increasing efficiency. However, they leave out the fact that this secondary burning cannot be achieved at temperatures below 1100.degree. F.
Examples of such stoves as discussed in the preceding paragraph are U.S. Pat. Nos. 2,190,343; 4,102,318; 4,228,783; 3,168,088 and 4,201,185.
The newest stoves on the market are advertising catalylitic converters, which lowers the temperature necessary for complete combustion of the CO and other volatiles. These stoves are very expensive and really only lower the necessary temperature to about 800.degree. F.
Modern air tight stoves allow a longer burning time of the wood by choking the air supply down to allow a slower burn. A slow burn causes the temperature inside the stove to lower, thus only worsening the objective--complete, efficient, clean combustion.
In contrast, consider how a bonfire burns. A bonfire gets extremely hot, emits almost no smoke, and little ash remains after all the wood is consumed. In terms of calories emitted per pound of wood burned, no stove can match the efficiency of a bonfire. Everything is totally consumed, the carbon, wood gases, tars, and other voliatile liquids. Everything except the water and ash, of course. Obviously there is a drawback, a large amount of heat is produced in a short period of time.